International Patent Application No. WO 93/15459 describes a bus access circuit. German Patent Nos. DE 36 03 750 and DE 36 03 751 describe bus segments and bus access circuits that are frequently used in programmable logic controllers having a modular structure.
In the case of such a programmable logic controller having a modular structure, it may be necessary to insert or withdraw assemblies in the course of operation. However, the insertion or withdrawal may give rise to the occurrence of feedback to the bus which connects the assemblies to one another, which feedback disturbs the bus traffic. In order to avoid such disturbances, provision is customarily made in the prior art for providing leading and/or lagging contacts on the assembly, which is provided with an evaluation circuit and a variable resistor (typically a MOSFET), with the result that the assembly is electrically connected to the bus only after complete insertion and is already electrically isolated from the bus before complete withdrawal.
This procedure is disadvantageous insofar as at least the evaluation circuit must be supplied with power from the outset. However, even the connection of the evaluation circuit to the power supply can already bring about disturbances to the voltage supply of the bus, to be precise particularly when a fault occurs in the evaluation circuit or the voltage supply of the evaluation circuit. A short circuit of the two supply contacts, for example, of the assembly, by means of which the evaluation circuit is supplied with power, would ineluctably lead to a complete collapse of the power supply of the entire bus. The same consequence would emerge if other, external effects on the plug-in location for the assembly caused the supply contacts of the plug-in location to be short-circuited to one another. This case may occur particularly when an assembly which, although it can be inserted mechanically into the plug-in location, is not, however, electrically and functionally intended for this plug-in location is inserted into the plug-in location.